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Article

“A Woman Clothed with the Sun”: The Diagnostic Study and Testing of Enzyme-Based Green Products for the Restoration of an Early 17th Century Wall Painting in the Palazzo Gallo in Bagnaia (Italy)

by
Claudia Colantonio
1,
Luca Lanteri
2,
Ramona Bocci
3,
Valeria Valentini
3 and
Claudia Pelosi
2,*
1
National Research Council, Institute of Complex Systems (CNR-ISC), University of Rome “Sapienza”, P.le Aldo Moro, 5, 00185 Rome, Italy
2
DEIM Department, Laboratory of Multispectral Imaging, University of Tuscia, Largo dell’Universtà, 01100 Viterbo, Italy
3
DIBAF Department, University of Tuscia, Largo dell’Università, 01100 Viterbo, Italy
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(23), 12884; https://doi.org/10.3390/app132312884
Submission received: 10 November 2023 / Revised: 27 November 2023 / Accepted: 29 November 2023 / Published: 30 November 2023
(This article belongs to the Special Issue Innovative Methods of Cleaning Polychrome Works of Art)
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Abstract

:

Featured Application

Use of enzymatic gels to clean wall paintings. Application of multispectral imaging to evaluate the effect of the cleaning operations on artwork surfaces.

Abstract

A 17th century wall painting, representing the Virgin between two Saints, in a noble Italian renaissance palace, the Palazzo Gallo in Bagnaia (Viterbo, Italy), was restored in 2021 in the context of a wider restoration campaign involving the main room of the palace built by cardinal Sansoni Riario. Diagnostic analyses performed using traditional characterization techniques (optical microscopy on micro-stratigraphic sections, X-ray fluorescence spectroscopy and Fourier transform infrared spectroscopy) provided the identification of both the original painting and its restoration materials, while imaging investigations using the ultraviolet fluorescence photography, false color images and multispectral mapping provided by the hypercolorimetric multispectral imaging (HMI) technique enabled the evaluation of the state of conservation, the location of restoration interventions and supported the monitoring of the cleaning procedure. An altered protective Paraloid®-based coating dating from the early 2000s had to be removed due to the unpleasant glossy finishing it had given to the painted surface, making the scene barely readable. To pursue a restoration protocol based on environmental sustainability and green chemistry, enzyme-based gels marketed by the Nasier-Brenta© and CTS© companies were tested in different protocols for the cleaning of the mixture (known as beverone) covering the painting. Although some interesting results were observed, the enzymatic cleaning had limited effectiveness, and was more timing-consuming than was reasonable. Traditional chemical solvents such as Dowanol PM (methoxy propanol) and benzyl alcohol were necessary to complete the cleaning of the painting surface.

1. Introduction

1.1. The Case Study

The rich, widespread, and complex artistic heritage of the Renaissance of central Italy represents an honorable charge and a challenging task for art historians and conservators, often overshadowed by the renowned opulence of the city of Rome. But the 16th–17th century Rome of the Popes delivered to history many splendid surroundings such as the villages of the Tuscia region around Viterbo, the seat of the first conclave in history.
One of these is surely Bagnaia, a suburb on the route of the Via Francigena which hosts the Palazzo Gallo, built between 1505 and 1521 by the will of the cardinal Raffaele Sansoni Riario [1,2,3] and later owned by noble Gallo family, richly frescoed both in its inner rooms and on its outer walls.
To the main wall decorations of the edifice, executed in all probability at the same time as the construction of the palace itself, were subsequently added further paintings, dated to the 17th century, which include the wall painting of the Virgin between Saints Sebastian and Saint Millerio (1.80 × 2.10 m2), the object of this study (Figure 1).
The painting is located on the west wall of the Sala Riario and both its artist and client remain, to this date, unknown. As with most paintings without an artist, the dating is uncertain, but stylistic examination suggests that it could be placed in the earliest two decades of the 17th century. A study focused on the observation of all the elements depicted within the painting and the way in which they are connected to each other has led to the identification of the theme as that of the Immaculate Conception rather than the Ascension, which was originally suggested by art historians [4].
In support of this attribution, in the Bagnaia wall painting, the Virgin stands barefoot on a crescent moon. Such iconography recalls a passage from John’s Apocalypse (Apocalypse 12, 1–3) referring to a “woman clothed with the sun” (mulier amicta sole). The crescent moon is indeed a recurrent symbol in the iconography of the immaculate conception, as testified by the artworks of important 17th century painters such as Guido Reni, Guercino and Federico Fiori, who was called Barocci [5,6].
The use of a comparative method was fundamental to the study of the wall painting, in which the compositional schemes and individual figurative details appearing in contemporary pictorial artworks were compared to this painting via analogy. Investigating the painters active in Bagnaia at that time, the greatest likeness, in terms of style and technique, was found with the work of the famous Roman painter Marzio Ganassini, who learned from the Mannerist school under Cavalier d’Arpino [7]. As evidence of his importance, in the years between the 16th and 17th century, Ganassini took part in the decoration of prestigious churches: a document of 1598 attests to him working on the church of Santa di Farfa, directed by Orazio Gentileschi; from 1591 to 1601 he worked in Sipicciano (province of Viterbo), painting the Baglioni Chapel in the church of Santa Maria Assunta in Cielo [8]; between 1601 and 1607 he lent his artistic skills to the decoration of the Roman churches of Santa Cecilia in Trastevere, Santa Maria della Consolazione and in cloister of Santa Maria sopra Minerva.

1.2. The Conservation History of the Wall Painting

The conservation history of the Virgin wall painting in the Palazzo Gallo is intertwined with the ones of paintings inside and outside the palace. Between 2003 and 2004, a restoration campaign directed by Professor Giuseppe Moro from Istituto Superiore per la Conservazione e il Restauro (ISCR) faced the removal of a firm coating and, using several substances such as potassium hydroxide, citric acid and acetic acid, which they delivered to the surface via a cellulose pulp pack. Unfortunately, this approach turned out to be aggressive towards the pictorial layer, which lost part of its original color. The last phases of Moro’s restoration saw the application of a protective layer alternating—in different and still unidentified areas—mastic, dammar, shellac, and various acrylic polymers, referred to as Paraloid® B-72 (a copolymer based on ethyl methacrylate and methyl acrylate), Primal® (without further specifications) and Acryl® AC33 [poly(ethyl acrylate/methyl methacrylate)] in nitro diluent (information given by Marcello Labate, a restorer on Moro’s team in 2004) [9].
The degradation of this protective layer is particularly noticeable in the lower part of the wall painting, where efflorescence phenomena also rise due to saline, and extensive abrasions and lacunae are predominant (Figure 2). Another main conservation issue for the painting is represented by the cleaning dowels from the former restoration, the most invasive of which is in the upper right area and includes the two putti and the face of the Virgin. Here the aggressive cleaning caused a thinning of the pictorial layer, lightening the tones of the paint with respect to its original state.
To conclude, due to the poor state of the conservation of the painting, the entire pictorial surface is covered by a protective layer that alters our perception of the painting, giving it a translucent appearance. The complex conservative framework represented a challenging restoration project but also the chance to test innovative and sustainable green products for the removal of the glossy protective layer, which was approached in this study.

1.3. The Aims of the Present Research

Considering the established potential toxicity of the traditional solvents used in the field of restoration, as well as in other application areas, scientific research is hopefully pushing towards the progressive elimination of substances which are potentially toxic for restorers and environment, choosing, where possible, the so-called “green” approach. Of the cutting-edge green technologies and products for the cleaning of painting surfaces, enzyme-based solutions have aroused considerable interest since the 1970s [10]. Enzymes are proteins, the catalysts of biochemical reactions, which have found successful application in heritage conservation due to their high selectivity towards the substrate; in the restoration field, this characteristic enables the selective removal of film-forming materials such as animal and fish glues and jellies, albumin and casein, starch, vegetable glues, drying oils, fats, some waxes, and some synthetic resins [11,12,13,14,15,16,17,18,19,20]. However, their high selectivity is also responsible for their limited operating range of effectiveness and their significant sensitivity to temperature and pH conditions. So, before applying this kind of cleaning system it is necessary to know the substrate’s composition and to establish the correct operative conditions.
For this reason, before starting restoration activities, a diagnostics campaign was performed in order to deepen our knowledge of the status of conservation and the materials of the artwork, both original and superimposed, as is usual in the field of cultural heritage objects [21,22].
Imaging techniques were firstly applied to study the overall surface of the painting, specifically, ultraviolet fluorescence photography (UVF) and hypercolorimetric multispectral imaging (HMI) were used at the beginning and at the end of the intervention to check the results of the restoration [23,24].
The initial use of these imaging techniques helped in choosing the areas for the spectroscopic analysis performed using portable and laboratory methods. The on-site analysis was carried out via X-ray fluorescence spectroscopy (XRF), which allowed for detecting the main chemical elements of the paintings, from which the pigment palette can be derived. Lastly, the non-invasive initial approach guided the choice of the sampling points on which to perform Fourier transform infrared spectroscopic (FTIR) analysis and the micro-stratigraphic investigation of the cross-section.

2. Materials and Methods

2.1. Hypercolorimetric Multispectral Imaging (HMI) and Ultraviolet Fluorescence Photography (UVF)

Multispectral images and UVF photographs were acquired through Hypercolorimetric Multispectral Imaging (HMI), a portable system developed by the Italian company Profilocolore© (Rome, Italy) consisting of a Nikon (Nital SpA, Moncalieri Torino, Italy) D800 36 Mpx camera, modified to obtain full-range light (300 to 1000 nm), 3 filters named A (UV-Vis acquisition), B (Vis-NIR acquisition) and UV-IR cut (sandwiched with filter A for UVF acquisition), standard white patches and a color-checker positioned in the scene [21,24,25]. The illuminating system for multispectral imaging is based on modified flashes. The UVF image was acquired using two CR230B-HP 10W UV LED projectors, with peak emission at 365 nm, mounted at 45° with respect to the camera [26].
The two acquired images, one with filter A and the other with filter B, are then calibrated through SpectraPick® (Version 1.2, created by Profilocolore, Rome, Italy) software, which produces seven high resolution spectral images in .tiff format from UV to NIR at 350, 450, 550, 650, 750, 850 and 950 nm and the RGB image. The calibration procedure allows for achieving a precision higher than 95% on the spectral reflectance images and a color error CIE ΔE2000 less than 2. After the calibration step, the images are uploaded in the processing software, PickViewer® (Version 1.0, created by Profilocolore, Rome, Italy), that provides several diagnostic imaging and statistical tools to produce false color images, apply principal component analysis (PCA) to different spectral bands to enhance hidden information and map materials based on their spectral reflectance or colorimetric values.

2.2. X-ray Fluorescence Spectroscopy (XRF)

XRF spectra were acquired by Dr. Claudio Falcucci (MIDA company, Rome, Italy) with a portable in-house-built XRF spectrometer operating with the following settings: Au tube at 40 kV, current at 0.04 mA and acquisition time equal to 50 s. The instrument was equipped with an Si-PIN detector (resolution 155 eV at 5.9 keV). XRF analyses were conducted on 20 points of the painted surface in order to characterize pigments of the original painting palette and eventual restoration and/or degradation products. The points of the XRF analysis are shown in Figure 3.

2.3. Laboratory Analysis of Micro-Samples

2.3.1. Fourier Transform Infrared Spectroscopy (FTIR)

Infrared spectroscopic analysis was performed to characterize the glossy restoration coating covering a large extent of the wall painting’s surface. The FTIR spectrum of sample S1 was acquired using a Nicolet Avatar 360 spectrometer with a DTGS detector (deuterated tri-glycine sulfate), the interferometer of Michelson and an accessory for diffuse reflectance analysis (DRIFT). About 5 mg of sample was used, mixed and ground with potassium bromide (KBr) of a spectrophotometric grade, which was used also as the background. A total of 128 spectra were acquired in the mid-infrared region (400–4000 cm−1) with a resolution of 4 cm−1. Spectra processing was performed in Thermo’s OMNIC version 8.0 software from Thermo Fisher Scientific (Waltham, MA, USA). The reading and interpretation of the spectra was based on the management tools available in above-mentioned software, scientific literature [27,28,29,30], online databases [31,32] and in-house libraries created by the Laboratory of Diagnostics and Materials Science of the University of Tuscia.

2.3.2. Microstratigraphic Analysis of the Cross-Sections

Five micro-samples (S2–S6) were selected and taken from the wall painting to prepare cross-sections for microscope examination, with marginal portions of the artwork chosen or those close to pre-existing lacunae. Sample cross-sections were prepared by LabSTONE snc (Palermo, Italy) after the micro-samples were embedded in a resin, suitably cut and polished. The prepared cross-section was observed via a polarized light microscope Zeiss Axioskop; equipped with reflected, transmitted and UV lighting. The images were acquired through Zeiss AxioCam NRc (Zeiss, Oberkochen, Germany) camera and processed using AxioVision software to trace the sequence of the stratigraphic layers and to recognize possible traces of organic material or the characteristic fluorescence of materials by means of observation via ultraviolet radiation.

2.4. Testing of Enzyme-Based Cleaning Products

In this work, enzyme-based gels produced by two Italian companies, Brenta (Milan, Italy) and CTS (Altavilla Vicentina, Vicenza, Italy), were tested on the wall painting of the Virgin to achieve the sustainable restoration of the artwork using innovative and environmentally friendly cleaning products.
The enzyme-based system from Brenta, called Nasier Lapideo L02, is an aqueous gel, ready to use, that was applied using a brush in two different test areas of the painting.
The CTS product must be prepared immediately before use. The CTS kit is composed of a solid enzyme mixture (containing lipase and esterase), a supporting agent named Klugel G (hydroxy-propyl-cellulose) and a washing solution to remove the residues from the surfaces at the end of the treatment.
The application times may be different depending on the conservation state of the surface and on the temperature of the environment.
Brenta suggests the times indicated in Table 1. The CTS technical data sheet suggests much lower times, from 1 to 10 min without dependence on the temperature: the right time should be selected after preliminary tests of the surfaces to be treated.
The products were applied via brush in two selected test areas, as subsequently reported in Figure 4. The areas chosen were in the marginal part of the painting in order to preserve the integrity of the main parts of the scene, such as the figures and the angels. The application times were generally higher than those proposed by Brenta and CTS. This was due to the poor results obtained with the times indicated in the technical data sheets, so, in accordance with the companies’ instructions, higher treatment times were tested.
Various phases were necessary due to the practical adjustments required for selecting the most suitable times and conditions. In fact, one important aspect of the work concerned the need to adapt the commercial products to a real case, because the suggestions given in the technical data sheets were not appliable to the wall painting object of the present study. As an example, the Nasier Lapideo L02 gel had a low viscosity and dripped onto the wall, so it was necessary to contact Brenta, which was able to design a new gel with higher viscosity.

3. Results

3.1. HMI and UVF Results

Before restoration, multispectral image investigations were performed using the HMI technique and UVF photography to analyze the general state of conservation of the pictorial layer and to identify the original painting materials. Imaging data were processed in the HMI software PickViewer® allowing us to fully detect the bruising, lacunae, grouting and plastering of former restorations, as well as to characterize the original painting materials through calibrated false color imaging analysis and map the pigments’ distribution through digital image processing algorithms.
The UVF image (Figure 5) was acquired before restoration to understand the general state of conservation of the wall painting and to highlight the presence of the residual restoration coating, whose fluorescence is clearly visible along the cracks of the pictorial layer and is more concentrated in some areas, such as on the body of the top right cherub, the left part of the Virgin’s mantle and a small portion of the sky under St. Sebastian’s bent knee.
False color images (Figure 6A) allow us to hypothesize the presence of smalt blue pigment for the Virgin’s mantle and for the sky painted in the lower part of the scene, as it typically becomes wine-red in IRFC and light green in UVFC images [23,33,34]. Another pigment that could be deduced, on the basis of its response to false color images, is vermilion/cinnabar, which appears bright yellow in IRFC and dark violet in UVFC (Figure 6B); it can be observed in the edges of the mantle of Saint Millerio.
To map the distribution of probable smalt blue and to further explore the potentiality of HMI processing software, the multispectral similarity tool and the cluster analysis (applied on the three IR bands) of PickViewer® were applied to the calibrated images by selecting a 3 × 3 pixel area in a blue zone of the Virgin’s dress. The results are shown in Figure 7A. The white pixels highlight the points where a pigment with similar spectral characteristics to those of the Virgin’s garment is present. The blue pigment appears more concentrated in the Virgin’s garment and in the background, but it is also present in other areas of the painting, such as in the mantle of St. Millerio and in the wings of the angel on the upper left side, as better highlighted by the cluster analysis, the result of which is shown in Figure 7B.

3.2. XRF Data

The results of the XRF analysis are summarized in Table 2, in which the detected elements are listed in order of decreasing abundance. The data revealed the joint presence of calcium (Ca), iron (Fe), rubidium (Rb), strontium (Sr) and zirconium (Zr), attributable to the constituent materials of the plaster, based on lime (to which calcium and strontium can be referred) and of a material of volcanic origin such as pozzolana (characterized by the presence of iron, rubidium and zirconium). The analysis is not able to identify the possible presence of an additional silicate filler (due to the impossibility of detecting silicon) or a carbonate one (the calcium of the charge would not be distinguishable from that of the binder). The ubiquitous presence of lead (Pb) could suggest the presence of this element in trace amounts in the plaster filler. Alternatively, the same element could be related to the use of modest quantities of lead white (or another lead-based pigment, for example minium/red lead) by the painter. In the blue points, a constant presence of arsenic (As), cobalt (Co), nickel (Ni) and bismuth (Bi) was detected, which allow us to identify the use of smalt. Residues of this pigment are also correspondingly present in the lacunae of the pictorial layer in the Virgin’s mantle.
The presence of Co, As and Ni is associated with the minerals used for producing blue smalt, such as smaltite (in the Middle Ages) and later erythrite and cobaltite (in the seventeenth and eighteenth centuries) [35]. According to some authors, the presence of bismuth associated with Co, As and Ni suggests that the minerals came from the Erzgebirge mines in Saxony [36,37].
In the red-colored areas, as well as for the red component of the skin tone of St. Millerio, the XRF analysis indicates the presence of mercury and iron, which indicates the use of vermilion/cinnabar (HgS) and earth pigments. On the other hand, vermilion/cinnabar seems absent from the red brushstrokes of the blood that gushes from the wounds on St. Sebastian’s legs, painted with red earth pigments. In the tracts of blood with a more orange and brilliant color, the increase in intensity of the lead fluorescence line and the simultaneous decrease of that of iron might suggest that minium was also employed. However, it is necessary to consider that XRF analysis cannot reveal the presence of organic dye pigments, which could have been used for the making of the blood.
At some points, XRF detected traces of arsenic, such as in the white in the crescent moon on which the Virgin stands, in the brightest yellow light of the mantle of San Millerio (whose color is attributable to yellow ocher) and in the dark part of the red robe of the Virgin. Here, the presence of arsenic could be due to the use of a very small quantity of smalt blue (for which cobalt, nickel and bismuth would be in quantities smaller than the detection limits of the instrument used in the configuration analysis), or to impurities of this element in some earth pigments, or to the use of orpiment (As2S3) and/or realgar (As4S4). The latter hypothesis seems, however, rather unlikely, considering the low photon counting rate related to arsenic, which is not sufficient to provide a chromatic perception in the painting.
Lastly, an arsenic contamination could also be traced back to the residues of sanitizing treatments for wooden elements, such as the extensive ones on the ceiling of the room hosting the wall painting.

3.3. Results of the Analysis of the Micro-Samples

3.3.1. FTIR Data

FTIR analysis was carried out on a sample (S1, localized in Figure 3) of the translucent finishing layer of the painting with the aim of characterizing the main constituents and choosing the most effective cleaning strategy.
The spectrum obtained is shown in Figure 8, together with the reference spectra of Paraloid® B44 and Paraloid® B72.
In fact, the research in the databases and in the texts reported in Section 2.3.1 resulted in a match with acrylic resins, specifically Paraloid® B72 and Paraloid® B44. Paraloid® B72 is a copolymer of ethyl methacrylate (EMA) and methyl acrylate (MA) with a ratio of 70/30 and the possible addition of butyl methacrylate (BMA) [38,39]. Paraloid® B44 is a copolymer of methacrylate (MMA) and ethyl acrylate (EA) in unknown proportions [39].
The main signatures of such polymers are 1725 cm−1 (C=O stretching); 2982, 2954 and 2929 cm−1 (stretching of C-H bonds); the 140–1300 cm−1 region (bending of the C-H bonds); and the 1300–900 cm−1 region (stretching of the C-O bonds). The presence of a synthetic resin of acrylic typology is thus confirmed, as mentioned by the restorer Marcello Labate who worked on the painting during the restoration campaign directed by prof. Moro in 2004.

3.3.2. Result of Microstratigraphic Analysis

The cross-sections, under visible reflected light and ultraviolet fluorescence, are shown in Figure 9. They were used mainly to investigate the painting technique in different parts of the wall painting. In all cases, the painting layer was well-adhered to the plaster, suggesting the use of a fresco technique. This assessment may be further confirmed by the absence of fluorescence in the painting layer, apart from the pale blue fluorescence that could be associated with calcium carbonate, the main constituent of the plaster and binder of the pigments. The painting layers have different thicknesses, sometimes, as in sample S5 (Figure 9G,H) of only few microns that, in fact, represent the preparatory drawing generally made using so-called sinopia, a red earth pigment based on iron oxides. In the cross-section of sample S5, the protective layer applied in the previous restoration is visible on the surface and exhibits an orange fluorescence. This same response is partially visible in the S1 and S6 samples’ cross-sections as well.
Sample S3, from the blue background in the lower part of the painting, clearly shows the smalt glassy particles applied using a fresco technique with a superimposed yellow layer that could be associated with the decorative element under the Virgin’s foot. This result suggests that the blue background was painted on the entire surface, at least in this portion of the wall painting, and the yellow decoration was applied after, directly on the blue smalt.
The layers’ superimposition is visible in the sample S2 cross-section, where the thin red layer, used to create the false marble decoration of the frame, is covered by a discontinuous black layer that could be sooty material fixed via the protective layer of the 2003–2004 restoration.
The sample S4 cross-section shows an thick orange layer adhering to the plaster and the two pinky and thick layers used to make the flesh tone.
The surface of sample S4’s cross-section is partially detached and appears discontinuous. This appearance can be explained by the hard cleaning treatment performed during the 2003–2004 restoration, clearly visible in the photo of the painting before our work (see Figure 1A, upper right side of the painting).

3.4. Results of the Cleaning Tests

After confirming the presence of Paraloid® through FT-IR analysis, we decided to test the green products from Nasier-Brenta for their cleaning of the painted surface. The company’s enzyme, Nasier Lapideo L02, as specified in the technical data sheet, is a ready-to-use product for stone surfaces which is specifically for the removal of organic patinas such as waxes, drying oils, oleo-resinous paints, repaintings and synthetic compounds such as acrylic and vinyl esters and Paraloid® B72.
In the first application, the aqueous gel based on the stabilized enzymes (lipases) had problems with adhering to the substrate due to its excessive fluidity, causing the solution to drip as soon as it was applied on the painting, thus preventing the correct functioning of the enzymes (Table 3). We then decided to contact the company, which provided a new sample specifically designed to overcome this problem (PHASE III in Table 3, Nasier L02 modified).
This second formulation clearly had a greater density than the first solution, but it was still not enough to stably cling to the surface. Following a further consultation with the company, it was decided that covering the gel with a layer of transparent film would hold the gel to the surface and avoid its possible evaporation. Unfortunately, even with these adjustments, the tests did not achieve the desired results, as no consistent improvement was registered.
We therefore decided to test the enzymatic cleaning mixture provided by CTS. As reported in their technical data sheet, the product is a mixture of purified lipase/esterase enzymes in a dehydrated form with a supporting solution containing Klucel G; it is specifically for the removal of natural waxes, synthetic resins such as esters, acrylics and vinyl and drying oils. The CTS product consists of one dose of enzyme in a powder form, a thickening dose for the supporting solution and a washing solution, which is an operating procedure very different from that developed for Nasier, which did not require any preparation. Nonetheless, CTS’s green product also did not meet expectations for a proper cleaning of the painting.
The results of the cleaning tests performed on selected areas of the wall panting are summarized in Table 3.

4. Discussion

Art restoration is a complex process in which different issues should converge equally: a proper recovery of the aesthetics and mechanical/physical conditions of the artwork; healthy working conditions for the restorers; limited chemical products and residual waste materials to dispose of at the end of the restoration; sustainable costs for a complete diagnostic study and restoration of the artwork; and, last but not least, timing compatible with the clients’ and the renewed fruition of the artwork. This ideal management of the restoration process often clashes with the fact that many conservation methodologies lack durability, sustainability, and cost-effectiveness and are typically based on energy-consuming processes or non-environmentally friendly materials.
In the current state of the art, there are still numerous conservation–restoration practices which are particularly challenging in terms of environmental protection; cleaning is by far the most crucial part of art restoration and a critical operation for ecological issues considering that it implies the use of solvents and other toxic chemicals. Nonetheless, a growing awareness has been registered in restoration field, in which environmental impact is earning more space in the scientific discussion of the effective cleaning protocols available [40]. Indeed, sustainable research and alternative materials are being developed and the sharing of good practices is spreading, together with a more solid acceptance of climate change and the environmental crisis due to plastics and pollutants which require long-term strategies to combat effectively.
Unfortunately, sometimes the green protocols successfully applied to a category of artworks are not effective for a specific case study, or not compatible with the needs of the restoration campaign. This is the case in the Palazzo Gallo’s wall painting presented in this study, for which the enzymatic products for stone cleaning by Nasier-Brenta and CTS could not provide appreciable results with acceptable timing, leading the restorers to choose a traditional chemical product, Dowanol, for the cleaning of the altered varnish. Dowanol PM (propylene glycol monomethyl ether or methoxy propanol) is a low-toxicity ether, with good penetrating power conferred by its low volatility and the fact that it is miscible with water. Considering its solubility parameters (Fd = 43; Fp = 20 and Fh = 37), it is considered suitable for the removal of natural resins [41].
A disappointing result does not compromise the path to an increasing awareness of waste management in restoration campaigns, which involves reducing the usage of chemicals and non-recyclable materials, but it should remind us that this path is still complex, and more efforts are needed to find solutions that can be customized to real cases. New green protocols need to be tested, with a focus on their monitoring and possible interface changes for the life cycle assessment of sustainable products, to propose effective, affordable, safe, and user-friendly solutions for restorers and conservators.

Author Contributions

Conceptualization, V.V., C.P. and L.L.; methodology, C.P. and V.V.; software, L.L., C.C. and R.B.; validation, C.P., L.L., R.B. and V.V.; formal analysis, L.L., C.C and C.P.; investigation, C.P., L.L. and C.C.; resources, C.P. and V.V.; data curation, R.B., C.C. and L.L.; writing—original draft preparation, C.C. and C.P.; writing—review and editing, all authors; visualization, C.P., C.C. and L.L.; supervision, C.P. and V.V.; project administration, C.P. and V.V.; funding acquisition, C.P. and V.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not appliable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank Brenta and CTS for their support and help during the testing of their commercial products, and Clauda Falcucci of the MIDA society for the XRF analysis. A special thanks goes to the “Amici di Bagnaia” Association that contributed greatly to the historical documentation of the painting and supported in the logistics around it.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 12884 g001
Figure 1. Calibrated RGB images obtained via HMI of the wall painting of the Virgin between Saints Sebastian and Millerio (1.80 × 2.10 m2) before (A) and after (B) the restoration.
Figure 1. Calibrated RGB images obtained via HMI of the wall painting of the Virgin between Saints Sebastian and Millerio (1.80 × 2.10 m2) before (A) and after (B) the restoration.
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Figure 2. Graphical representation of the state of conservation of the wall painting before restoration (A) and a mapping of former interventions on the pictorial layer (B).
Figure 2. Graphical representation of the state of conservation of the wall painting before restoration (A) and a mapping of former interventions on the pictorial layer (B).
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Figure 3. Photograph of the painting before restoration with the points of the XRF analysis (X1–X20) in white and the sampling points in red (S1–S6).
Figure 3. Photograph of the painting before restoration with the points of the XRF analysis (X1–X20) in white and the sampling points in red (S1–S6).
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Figure 4. Summary outline of the test phases and areas of application of Nasier-Brenta and CTS enzymatic products.
Figure 4. Summary outline of the test phases and areas of application of Nasier-Brenta and CTS enzymatic products.
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Figure 5. UVF photography acquired via the HMI technique before the restoration.
Figure 5. UVF photography acquired via the HMI technique before the restoration.
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Figure 6. IRFC (A) and UVFC (B) images produced using PickViewer®-specific tools.
Figure 6. IRFC (A) and UVFC (B) images produced using PickViewer®-specific tools.
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Figure 7. Result of the multispectral similarity tool (A) and cluster analysis applied to the IR1, IR2 and IR3 bands (B).
Figure 7. Result of the multispectral similarity tool (A) and cluster analysis applied to the IR1, IR2 and IR3 bands (B).
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Figure 8. FTIR spectrum of sample S1.
Figure 8. FTIR spectrum of sample S1.
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Figure 9. Microphotographs of the cross-sections of samples S2 (A,B), S3 (C,D), S4 (E,F), S5 (G,H) and S6 (I,J) under reflected visible lighting (A,C,E,G,I) and ultraviolet fluorescence (B,D,F,H,J).
Figure 9. Microphotographs of the cross-sections of samples S2 (A,B), S3 (C,D), S4 (E,F), S5 (G,H) and S6 (I,J) under reflected visible lighting (A,C,E,G,I) and ultraviolet fluorescence (B,D,F,H,J).
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Table 1. Application times suggested by Brenta according to the environmental temperature.
Table 1. Application times suggested by Brenta according to the environmental temperature.
Condition of The SurfaceT < 12–15 °C12–15 °C < T < 25 °CT > 25 °C
Well-preserved surface90 min45 min45 min + PVC pellicula
Not well-preserved surface15 min or higher15 min or higher15 min or higher + PVC pellicula
Table 2. Chemical elements detected through XRF, reported in descending order of X-ray counts.
Table 2. Chemical elements detected through XRF, reported in descending order of X-ray counts.
PointColorDetected ElementsHypothesized Pigments
X1Blue (clear)As, Sr, Fe, Ca, Rb, Co, Bi, Zr, PbSmalt blue + lead white
X2Blue (clear)As, Fe, Sr, Co, Ca, Bi, Zr, Ni, Rb, PbSmalt blue + lead white
X3Blue (clear)Fe, As, Sr, Ca, Co, Rb, Bi, Ni, Pb, ZrSmalt blue + earth pigments + lead white
X4WhiteCa, Sr, Fe, As, Rb, Zr, PbCalcium carbonate + lead white
X5WhiteCa, Sr, Rb, Zr, Fe, PbCalcium carbonate + lead white
X6YellowFe, Ca, Sr, Rb, Zr, PbCalcium carbonate + yellow ochre
X7YellowFe, Ca, Sr, Rb, Zr, PbCalcium carbonate + yellow ochre
X8YellowFe, Ca, Sr, Rb, PbCalcium carbonate + yellow ochre
X9YellowFe, Sr, Ca, Rb, Pb, AsYellow ochre
X10RedFe, Ca, Sr, Hg, Rb, PbRed earth + cinnabar
X11RedFe, Hg, Ca, Sr, Rb, As, PbRed earth + cinnabar
X12WhiteCa, As, Sr, Pb, Bi, Fe, Zr, Co, Rb, NiSmalt blue + lead white + earth pigments
X13GreenFe, Sr, Ca, As, Pb, Rb, Zr, Co, Bi, NiEarth pigment + lead white + smalt blue
X14GreenFe, Sr, Ca, Rb, Zr, PbEarth pigment + lead white
X15Pink (clear)Ca, Fe, Sr, Rb, PbCalcium carbonate + earth pigments + lead white
X16PinkCa, Fe, Sr, Hg, Rb, PbEarth pigments + cinnabar + lead white
X17BrownFe, Ca, Sr, Rb, Hg, Mn, PbRed earth + cinnabar + umber
X18RedCa, Sr, Fe, Rb, Zr, Pb, MnRed earth + cinnabar + umber
X19RedFe, Sr, Ca, Zr, Rb, PbRed earth + lead white or minimum
X20Blue (clear)Sr, Fe, Ca, Zr, Rb, As, Co, Pb, Ni, BiSmalt blue + lead white
Table 3. Summary of the cleaning tests performed on the painting using the enzymatic mixtures.
Table 3. Summary of the cleaning tests performed on the painting using the enzymatic mixtures.
Test NumberEnzyme MixtureApplication ModalityVisible Result
PHASE I–TEST INasier L02By brush, on the surface for 20 min, inserting a sheet of Japanese paper between the surface and the enzyme mixtureNo result
PHASE I–TEST IINasier L02By brush, directly on the surface for 45 min without paperNo result
PHASE I-TEST IIINasier L02By brush, on the surface for 90 min, covering the enzymatic gel with transparent filmPartial removal is visible
PHASE II–TEST INasier L02By brush, directly on the surface and covered with Japanese paper and transparent film for 90 minNo result, the mixture is too fluid and drips onto the wall
PHASE II–TEST IINasier L02The same as test IThe same result as test I
PHASE II–TEST IIINasier L02The same as test IThe same result as test I
PHASE III–TEST INaser L02 modifiedBy brush, on the surface for 90 min, covering the enzymatic gel with transparent film—first applicationNo result
PHASE III–TEST IINasier L02 modifiedBy brush, on the surface for 90 min, covering the enzymatic gel with transparent film—second applicationPartial removal of the surface material
PHASE III–TEST IIINasier L02 modifiedBy brush, on the surface for 90 min, covering the enzymatic gel with transparent film—third applicationThe removal of the surface material is more evident than in Test II, second application
PHASE IV–TEST ICTS productBy brush, for 5 minNo result
PHASE IV–TEST IICTS productBy brush, for 10 minNo result
PHASE IV–TEST IIICTS productBy brush, for 45 minPartial removal of the surface material
PHASE IV–TEST IVCTS productBy brush, for 60 minPartial removal of the surface material
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Colantonio, C.; Lanteri, L.; Bocci, R.; Valentini, V.; Pelosi, C. “A Woman Clothed with the Sun”: The Diagnostic Study and Testing of Enzyme-Based Green Products for the Restoration of an Early 17th Century Wall Painting in the Palazzo Gallo in Bagnaia (Italy). Appl. Sci. 2023, 13, 12884. https://doi.org/10.3390/app132312884

AMA Style

Colantonio C, Lanteri L, Bocci R, Valentini V, Pelosi C. “A Woman Clothed with the Sun”: The Diagnostic Study and Testing of Enzyme-Based Green Products for the Restoration of an Early 17th Century Wall Painting in the Palazzo Gallo in Bagnaia (Italy). Applied Sciences. 2023; 13(23):12884. https://doi.org/10.3390/app132312884

Chicago/Turabian Style

Colantonio, Claudia, Luca Lanteri, Ramona Bocci, Valeria Valentini, and Claudia Pelosi. 2023. "“A Woman Clothed with the Sun”: The Diagnostic Study and Testing of Enzyme-Based Green Products for the Restoration of an Early 17th Century Wall Painting in the Palazzo Gallo in Bagnaia (Italy)" Applied Sciences 13, no. 23: 12884. https://doi.org/10.3390/app132312884

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